CHARACTERISTICS AND ORIGIN OF HIGH-GRADE METAMORPHISM OF THE PENNSYLVANIAN MINTURN FORMATION IN THE SANGRE DE CRISTO MOUNTAINS, SOUTHERN COLORADO

The Sangre de Cristo Mountains in southern Colorado expose variably metamorphosed sedimentary rocks of the Pennsylvanian Minturn Formation. In the Garner Creek area near Crestone previous studies document an anomalous zone of andalusite-biotite phyllite, but the timing, origin, and conditions of this metamorphism are poorly understood. We use field mapping, petrography, and Raman spectroscopy of carbonaceous material (RSCM) to characterize this metamorphic zone and understand its significance. Within the zone we identify a metapelitic mineral assemblage of andalusite-staurolite-biotite. Andalusite and staurolite porphyroblasts are largely retrogressed to chlorite and white mica, and quartz is statically recrystallized. This metamorphism is restricted to a 1.0-1.4 km² area. RSCM from 5 samples within this zone record peak temperatures of 460-540°C, while a sample ~180 m outside this zone records peak temperatures of 420-450°C. Leucogranite dikes are common within the zone, and metamorphic fabrics are locally more developed adjacent to dikes. These observations and the small size of the metamorphic zone strongly suggest that metamorphism is contact in origin and related to a leucogranite stock at depth. An exposure of leucogranite on the margin of the aureole has weighted mean zircon U-Pb date of 33 ± 1 Ma, placing metamorphism in the early Oligocene. Biotite and andalusite are locally oriented at a high angle to bedding and older foliation and are axial planar to subtle crenulations that record NW-SE shortening. The aureole is also characterized by abundant quartz veins and quartz-coated shear fractures that have variable orientations and record a heterogenous strain field. Quartz slickenfibers are statically recrystallized, indicating that metamorphism was coeval with brittle deformation. Metasedimentary rocks in the Garner Creek area represent some of the highest-grade Phanerozoic strata in the Rocky Mountain region, and during the Oligocene the Sangre de Cristo Mountains were locally much more heated than suggested by previous studies. The western flank of the range records significant lateral thermal gradients with ~500°C metamorphism near Garner Creek located only ~9 km north of Pennsylvanian strata with a Permian zircon fission track date, where peak temperatures likely never exceeded 250°C.